HONG KONG — Sometime in the next few months, a single-engine Cessna will fly from Sydney to London. Converted to be able to carry extra amounts of fuel, the small plane will take 10 days for its journey, making 10 or so stops along the way.

What will make this journey special is not the route or the identity of the pilot — a 41-year-old British insurance industry executive who lives in Australia — but the fuel that the aircraft will be using: diesel processed from discarded plastic trash.